Method of using fuel gas expander in power generating plants

ABSTRACT

In a power generating plant which utilizes fuel gas for combustion at a predetermined pressure to drive a primary load, and where the fuel gas is supplied at a pressure higher than the predetermined pressure, an improvement is provided wherein a fuel gas expander is located downstream of a source of the fuel gas and upstream of a combustor for decreasing the pressure of the fuel gas, and wherein excess energy from the expander is used to drive a secondary load.

TECHNICAL FIELD

This invention relates to power generating plants generally, and morespecifically, to the use of a fuel gas expander for reducing fuel gaspressure in a power generating plant.

BACKGROUND

Fuel gas used in power generating plants is available, in manysituations, at much higher pressure than is needed for combustion in thepower generating equipment. The source of high pressure fuel gas forpower generation may be a natural gas pipeline, pressurized fuelconversion gasifier, a chemical process plant, or any other highpressure combustible gas generating equipment. Usually, the fuel gaspressure needs to be reduced and controlled to the level required forstable combustion in the power generating combustion equipment, such asthe combustor in a gas turbine generator, or a fuel burner in a wasteheat boiler and fully fired boiler turbine generator, etc. Typically,pressure reduction is achieved by throttling the fuel gas, but this isnot particularly efficient since it does not make use of the fuel gaspressure differential.

DISCLOSURE OF THE INVENTION

In accordance with this invention, a fuel gas expander device isutilized in a power generation plant, and utilizes the differencebetween the fuel gas source pressure and the combustor/burner pressurein the power generating plant, resulting in improved power output andefficiency of the plant.

In one exemplary embodiment, one or more fuel gas expanders are used ina gas turbine, combined cycle power generating plant. Specifically, thefuel gas from a high pressure fuel gas source is introduced into a fuelgas expander prior to introduction into the combustor of the gasturbine. The excess pressure between the high pressure fuel gas and thelower pressure fuel gas exiting the fuel gas expander may be employed todrive a load such as a generator, compressor or other component.Optionally, the fuel gas can be cooled and/or conditioned if requiredprior to introduction into the fuel gas expander in order to remove anysolid particles or harmful contaminants which may adversely effect thecontinued operation and/or reduce the blade life of the expander.

The fuel gas can then, also optionally, be heated in a heat exchanger toa temperature higher than the minimum required to avoid any undesirablefluid condensation or deposition in the expander equipment. The sourceof this heat can be thermal energy recovered either upstream (forexample, from a gasifier with a high temperature cooler) or downstream(for example, from the exhaust gas heat recovered from the gas turbineexhaust in a waste heat boiler) of the expander.

The fuel gas may be expanded further in a second fuel gas expander ifthe fuel is to be used at two pressure levels, for example, where it isfirst expanded to an intermediate pressure level suitable for the gasturbine combustor, with a portion further expanded to an even lowerpressure in a separate fuel gas expander prior to introduction as asupplemental fuel in the fuel burner of a waste heat or fully firedboiler.

It is further contemplated that a portion of the fuel gas may bebypassed around the first fuel gas expander via a throttling device tocombine with the discharge stream from the expander to continue plantoperation during start-up, shutdown, or during other events when theexpander is not operating.

In another exemplary embodiment of the invention, a similar fuel gasexpander setup is employed in connection with a fuel burner in a fullyfired boiler turbo-expander (typically steam) power plant.

In its broader aspects therefore, the present invention provides, in apower generating plant which utilizes fuel gas for combustion at apredetermined pressure to drive a primary load, and where the fuel gasis supplied at a pressure higher than the predetermined pressure, animprovement including at least one fuel gas expander located downstreamof a source of the fuel gas and upstream of a combustor, the fuel gasexpander decreasing the pressure of the fuel gas below the higherpressure; and wherein excess energy resulting from a differentialbetween the predetermined pressure and the higher pressure is used todrive a secondary load.

Other objects and advantages of the subject invention will becomeapparent from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the use of a fuel gasexpander in a gas turbine/combined cycle power generating plant inaccordance with the first exemplary embodiment of the invention;

FIG. 1A is a schematic diagram illustrating a variation in thearrangement shown in FIG. 1; and

FIG. 2 is a schema tic diagram of a fuel gas expander in a fully firedboiler turbo-expander power plant in accordance with a second exemplaryembodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, a combined cycle gas/steam turbine system 10is sh own in schematic form. Generally, the gas turbine 12 includes acompressor section 14, a combustor 16 and a turbine section 18, thelatter driving a generator 20. Exhaust gas from the turbine 18 issupplied to a waste heat recovery steam generator 22 which suppliesheated steam to a steam turbine 24 which drives a second generator 26.

In accordance with this invention, the fuel gas from a high pressurefuel gas source 28 is reduced in pressure in a fuel gas expander 34prior to introduction into the combustor 16 of the gas turbine 12.Depending on particular circumstances, the fuel gas may be introducedinto gas cooling and/or gas conditioning equipment 30, employed, ifnecessary, to remove any solid particles and/or contaminants from thefuel gas prior to introduction into the fuel gas expander 34. Someportion of the fuel gas may be extracted from the conditioning equipment30 if desired. The fuel gas may also (again, optionally) be pre-heatedin an exchanger 32, also upstream of the expander. The pre-heater 32 orexchanger can receive heat energy for the exchange from a sourceupstream of the expander 34 such as the cooling and/or conditioningequipment 30, o r from a source downstream of the expander 34, such asthe optional gas cooling/conditioning equipment 38 (discussed furtherbelow), or from exhaust gas heat recovered from the gas turbine exhaustin the waste heat recovery generator 22. In some cases, the source ofheat will include partial conversion of fuel gas to chemical energy (forexample, thermal energy produced during shift reaction of synthetic fuelgas containing carbon monoxide and water vapors to carbon dioxide andhydrogen).

In the expander 34, the fuel gas pressure is reduced to a level requiredby the gas turbine 12, and excess energy (the difference between thefuel gas source pressure and the combustor/burner pressure) is used todrive a secondary load (a generator or other equipment) 36. Optionally,a portion of fuel gas may be extracted from the expander and exported toa process application.

With further reference to FIG. 1, the fuel gas may also be expanded in asecond expander 42 downstream of the first expander 34. As in the caseof the first expander, the fuel gas may flow through the second gascooling/conditioning component 38, and/or a pre-heater 40 prior tointroduction into the expander 42. This arrangement is useful where fuelgas is used at two pressure levels, i.e., where it is first expanded toan intermediate pressure level in the expander 34, suitable forcombustion in the gas turbine 12, with a portion of the expanded fuelgas further expanded to an even lower pressure in the second fuel gasexpander 42, for introduction as a supplemental fuel in the fuel burner44 of the waste heat recovery generator 22, via fuel control valve 46.Excess energy from the second expander 40 may then be used to drive atertiary load 48.

The pre-heater 40, if used, may be in heat exchange relationship withthe cooling/conditioning equipment 38 and/or the waste heat recoverygenerator 22, in a manner similar to that of the pre-heater 32 asdescribed above. The main portion of the fuel gas is introduced into thegas turbine combustor 16 via fuel control valve 50.

FIG. 1 also illustrates an optional bypass where some fuel can bediverted upstream of the expander 34 (and pre-heater 32 if used) by wayof bypass valve 52, for introduction into the discharge stream from theexpander 34 via fuel control valve 54. This bypass arrangement enablesplant operation during startup, shut down, and other events where theexpander 34 is not operating.

FIG. 1A illustrates an alternative arrangement where reduced pressurefuel gas controlled by valve 150 (corresponding to valve 50 in FIG. 1)is introduced into a lower pressure gas turbine combustor 116A of amulti-combustion (reheat) gas turbine cycle which also includecompressor 114, turbine sections 118A and 118B and a second combustor116B, with the reheat turbine driving a generator 120. If necessary, athird fuel gas expander may be incorporated into the system upstream ofthe control valve 152 with or without additional extractions, dependingon specific process applications.

Turning now to FIG. 2, a second embodiment of the invention isschematically shown which is generally similar to that shown in FIG. 1up to and including the gas cooling and/or conditioning equipment 38′.For convenience, therefore, similar reference numerals with a “prime”designation added, are used to indicate corresponding components, whichneed not be described again here. In this second embodiment, however,fuel gas from the equipment 38′ is introduced via flow control throttle56 to a fuel burner 58 in a fully fired boiler 60 which supplies steamto a turboexpander 62 which, in turn, drives a generator or other load64. The preheater 32′ can receive heat energy for the exchange from asource upstream of the expander 34′ such as the cooling and/orconditioning equipment 30, or from a source downstream of the expander34′, such as the optional gas cooling/conditioning equipment 38′ or fromexhaust gas heat recovered from fully fired boiler 60.

It will be appreciated from the above described embodiments of theinvention that excess fuel gas pressure, over and above that which isnecessary for its principal intended use, may be employed to drive otherrelated equipment in a power generating plant, thereby improving theoverall efficiency of the system.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A power generating plant comprising a compressorcomponent, a combustor and a gas turbine component, and which utilizesfuel gas for combustion at a predetermined pressure to drive a primaryload, and where the fuel gas is supplied at a pressure higher than thepredetermined pressure; and a fuel flow arrangement comprising at leastone fuel gas expander located downstream of a source of the fuel gas andupstream of the combustor, said fuel gas expander decreasing thepressure of the fuel gas below said higher pressure for use in saidcombustor, and wherein excess energy resulting from a differentialbetween said predetermined pressure and said higher pressure is used todrive a secondary load that is otherwise independent of the power cycle.2. The power generating plant of claim 1 wherein the power generatingplant is a combined cycle power generating plant and further whereinexhaust gas from the gas turbine component is introduced into a wasteheat recovery steam generator and then supplied to a steam turbine. 3.The power generating plant of claim 2 wherein the fuel gas is suppliedto a gas cooling component upstream of the gas expander.
 4. The powergenerating plant of claim 2 wherein the fuel gas is supplied to a gasconditioning component upstream of the gas expander.
 5. The powergenerating plant of claim 2 wherein the fuel gas is introduced into apre-heater upstream of the gas expander.
 6. The power generating plantof claim 2 wherein the portion of fuel gas is extracted from theexpander at desired pressure for export to a process application.
 7. Thepower generating plant of claim 2 wherein a portion of the fuel gas fromthe gas expander is supplied to a second gas expander to further reducethe pressure of the fuel gas.
 8. The power generating plant of claim 7wherein fuel gas from the gas expander is supplied to another gascooling component upstream of said second gas expander.
 9. The powergenerating plant of claim 7 wherein fuel gas from the gas expander issupplied to another gas conditioning component upstream of said secondgas expander.
 10. The power generating plant of claim 7 wherein saidportion of the fuel gas is introduced into another pre-heater upstreamof said second gas expander.
 11. The power generating plant of claim 7wherein excess energy from said second expander is used to drive atertiary load.
 12. A combined cycle power generating plant comprising acompressor component, a combustor, a gas turbine component, a waste heatrecovery generator and a steam turbine, wherein exhaust gas from the gasturbine is introduced into the heat recovery generator and then suppliedto the steam turbine; the combustor utilizing fuel gas for combustion ata predetermined pressure to drive a primary load, the fuel gas beingsupplied at a pressure higher than the predetermined pressure; and, afuel flow arrangement comprising at least one fuel gas expander locateddownstream of a source of the fuel gas and upstream of the combustor,said fuel gas expander decreasing the pressure of the fuel gas belowsaid higher pressure for use in said combustor, wherein excess energyresulting from a differential between said predetermined pressure andsaid higher pressure is used to drive a secondary load; and furtherwherein the fuel gas is introduced into a pre-heater upstream of the gasexpander, said pre-heater being in heat exchange relationship with saidwaste heat recovery generator.
 13. A combined cycle power generatingplant comprising a compressor component, a combustor, a gas turbinecomponent, a waste heat recovery generator and a steam turbine, whereinexhaust gas from the gas turbine is introduced into the waste heatrecovery generator and then supplied to the steam turbine; the combustorutilizing fuel gas for combustion at a predetermined pressure to drive aprimary load, the fuel gas being supplied at a pressure higher than thepredetermined pressure; and, a fuel flow arrangement comprising at leastone fuel gas expander located downstream of a source of the fuel gas andupstream of the combustor, said fuel gas expander decreasing thepressure of the fuel gas below said higher pressure for use in saidcombustor, wherein excess energy resulting from a differential betweensaid predetermined pressure and said higher pressure is used to drive asecondary load; and further wherein the fuel gas is introduced into apre-heater upstream of the gas expander, said pre-heater being in heatexchange relationship with a gas cooling component located upstream ofthe fuel gas expander.
 14. A combined cycle power generating plantcomprising a compressor component, a combustor, a gas turbine component,a heat recovery steam generator and a steam turbine, wherein exhaust gasfrom the gas turbine is introduced into the waste heat recoverygenerator and then supplied to the steam turbine; the combustorutilizing fuel gas for combustion at a predetermined pressure to drive aprimary load, fuel gas being supplied at a pressure higher than thepredetermined pressure; and, a fuel flow arrangement comprising at leastone fuel gas expander located downstream of a source of the fuel gas andupstream of the combustor, said fuel gas expander decreasing thepressure of the fuel gas below said higher pressure for use in saidcombustor, wherein excess energy resulting from a differential betweensaid predetermined pressure and said higher pressure is used to drive asecondary load; and further wherein the fuel gas is introduced into apre-heater upstream of the gas expander; said pre-heater being in heatexchange relationship with a gas cooling component located downstream ofthe fuel gas expander.
 15. The power generating plant of claim 2 andincluding a fuel gas bypass around said fuel gas expander.
 16. The powergenerating plant of claim 10 wherein the fuel gas is supplied to a gasconditioning component upstream of the gas expander.